Dynode structure and array for an electron multiplier

ABSTRACT

The electron multiplier is of the kind in which a charge current is amplified by successive passage to and secondary emission of electrons from dynodes which are arranged in two opposed rows. The multiplier is arranged for application of electric charge to the dynodes so as to focus the charge current on to each of the dynodes in succession, alternating between the rows. The dynodes have electron emissive surfaces to which the charge current passes and angled flanges located at opposite edges of the surfaces. The surfaces are preferably formed by aluminum foil. The rows of dynodes are preferably formed by aluminum foil. The rows of dynodes are supported on opposed cantilevered insulating members by crimped straps.

BACKGROUND OF THE INVENTION

This invention relates to electron multipliers of the kind in which acharge current is amplified by passage to and by secondary emission ofelectrons from surfaces of a dynode array, and to dynode arrays for suchelectron multipliers. In one aspect, the invention is particularlyconcerned with electron multipliers in which there are two generallyparallel rows of said dynodes in said array, the dynodes in each rowbeing in side-by-side position, and said successive dynodes beingdynodes in alternating ones of said rows, each said dynode being shapedsuch that electric potentials which are applied to the dynodes of thearray generate an electric field between the two rows such as to effectsubstantial direction of secondary electrons produced at each saidsurface to the surface of the next successive dynode; in which saiddynodes in one said row have the said surface thereof facing the othersaid row of dynodes and those of the other said row have the saidsurface thereof facing said one row; each said dynode having first andsecond flanges bounding respective opposed margins thereof, said firstand second flanges extending transverse to the directions of extent ofsaid rows, the flanges of dynodes in said one row extending from thesurfaces of those dynodes towards the other said row and the flanges ofdynodes of said other row extending from the surfaces of those dynodestowards said one row, and each adjacent pair of said dynodes in said onerow and in said other row having the first flange of one dynode of thepair adjacent and spaced from the second flange of the other dynode ofthe pair, and the first flange of each said dynode being closer to aninput end of the array than the second flange of that dynode; thesurfaces of dynodes in said one row and of dynodes in said other rowbeing linear and parallel to the lengthwise direction of extent of therespective row, when the array is viewed in lengthwise cross-sectionnormal to tangents to said surfaces at the location of thecross-section.

Our Australian Patent Specifications AU 39194/78 and AU 87312/75describe electron multipliers of the above kind. In these constructions,the aforementioned first flange is of generally L-shaped configurationhaving a first portion which extends normally to the respective dynodesurface and an outwardly extending second portion which extends awayfrom the edge of the first portion remote from the dynode surface in aplane parallel to the dynode surface. The second flange extends normallyfrom the associated dynode surface.

While the arrangements of patent specifications AU 39194/78 and AU87312/75 have proven to be highly satisfactory in use, the two portionform of the first flanges renders the manufacture of the dynodes lesssimple than would be desirable. In accordance with a first aspect of theinvention, this difficulty is avoided in that the first flange of eachsaid dynode extends at an obtuse angle to the surface of that dynode, toextend away from that surface towards said input end, and the secondflange of each said dynode extends at an acute angle to the surface ofthat dynode, from the junction of the flange with that surface, towardssaid input end.

In order to secure effective operation of electron multipliers, thesurfaces of the dynodes to which the charge current is directed must beselected so as to have good emissive characteristics in the sense that ahigh secondary electron yield is obtained on incidence of an electron.

The performance of emissive surfaces deteriorates during long term useof dynodes, so that the lifetime of an electron multiplier has been,hitherto, usually limited to the lifetime of the emissive surfaces. InAustralian Patent Specification AU 39194/78, a construction ofcylindrical multiplier is disclosed in which the emissive surfaces aredefined on strip metal which is removable from the dynodes so as toprovide a construction where the life of the multiplier can be renderedindefinite by replacement of the strips as necessary. However, it isnecessary in that construction to pre-form the strips to a specificconfiguration, such as by some suitable hand or machine processinvolving cutting as well as bending. Also the method of retention ofthe strips has rendered their replacement less simple than would bedesirable. In another aspect, then, the invention seeks to provide aconstruction which, whilst providing the advantages of removable dynodesurfaces, permits of a simplified removal and replacement procedure. Inthis aspect, then, the invention provides a dynode array for an electronmultiplier in which a charge current is amplified by passage to andsecondary emission of electrons from surfaces of successive dynodes ofthe array, characterised in that the emission surfaces of the dynodesare defined by foils removably positioned on supporting portions of therespective dynode. In a preferred form of the invention the foils arealuminum foils which have, unexpectedly, been discovered to have goodsecondary emission characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example only with referenceto the accompanying drawings in which:

FIG. 1 is a side view of an electron multiplier constructed inaccordance with this invention;

FIG. 2 is a cross-section substantially on the line 2--2 in FIG. 1;

FIG. 3 is a top view of the multiplier of FIG. 1;

FIG. 4 is a circuit diagram showing electrical interconnections made tothe multiplier of FIG. 1 in use;

FIG. 5 is a perspective view of a dynode incorporated into themultiplier of FIG. 1;

FIG. 6 is a scrap perspective view showing the manner of attachment ofdynodes in the multiplier of FIG. 1;

FIG. 7 is an electron trajectory diagram illustrating the performance ofthe multiplier of FIG. 1;

FIG. 8 is an axially sectioned view of a modified electron multiplierconstructed in accordance with the invention;

FIG. 9 is a transverse cross-section of a dynode like that shown in FIG.5, but having the emissive surface thereof defined by a removable foil;

FIG. 10 is a perspective view of the dynode of FIG. 9;

FIG. 11 is a diagram illustrative of further modification of theinvention; and

FIG. 12 is a graph illustrating the performance of various dynodeemissive surfaces.

DETAILED DESCRIPTION OF THE INVENTION

The electron multiplier 10 of FIGS. 1 to 3 has a dynode array comprisedof two rows 12, 14 of dynodes which rows extend generally parallel toeach other from an input end 10a of the multiplier to an output end 10b.Aside from end dynodes described later, the dynodes in each row aregenerally similar being of the form of the dynode 16 illustrated in FIG.5. More particularly, each dynode 16 has a planar portion 17 defining aplanar secondary emission surface 18 of rectangular form and first andsecond flanges 20, 22 which extend along opposed side margins of thesurface 18. Flange 22 extends at an acute angle to surface 18, beingreversely bent to overlie the surface 18. Flange 20 is bent so as toform an obtuse angle relative to surface 18 and is generally parallel toflange 22. As shown, each flange is at an angle of 45° to the directionof extent of the surface 18 when viewed from the side as shown forexample in FIG. 1. The flange 20 is somewhat larger than flange 22,being longer as viewed from the side such as in FIG. 1. The dynodes 16may be formed from a rectangular blank of sheet metal by simple foldingoperations to form the flanges 20, 22 and portion 17.

Except for an input dynode 46 described later, the dynodes 16 in row 12are supported on an insulating member 32 by means of thin metal straps28 secured to reverse faces of the dynode portions 17. The dynodes 16 inrow 14 are likewise assembled in analagous fashion on to a supportinginsulating member 26, which member 26 also carries dynode 46.

The members 26 and 32 are formed from insulating material such asalumina, being of rectangular planar elongate form. As shown in FIG. 6member 32 has a series of cut-outs or notches 34 formed in the sideedges thereof, the notches being aligned in opposed pairs along thelength of the member. The member 26 is similar, having a similar seriesof notches 34 in the side edges. The dynodes 16 are supported on therespective member 26 or 32 with the reverse surfaces of portions 17parallel to and overlying one major face 26a, 32a of the respectivemember 26 or 32. The straps 28 are thus positioned between the reversefaces of the dynode portions 17 and the respective member 26 or 32.Opposed arm portions 28a, 28b of the straps 28 thence extendtransversely across the major face 26a or 32a of the respective member26, 32 into the notches 34 and thence along outer faces 26b, 32b of themembers 26, 32. At the outer faces, the opposed ends of each strap arecrimped and spot-welded together in a fashion such as to apply tensionto the arm portions 28a, 28b to tightly hold the dynodes in position.

The notches 34 are of width, measured in the lengthwise direction of themembers 26, 32 only slightly greater than the width of the straps 28 andthus serve to accurately locate the straps and affixed dynodes inposition. This method of affixment results in the dynode surface 18 ofeach dynode being maintained with a high degree of accuracy generallyparallel to the direction of extent of the members 26, 32 and providesfor accurate spacing of the dynodes along the lengths of the members.

As best shown in FIG. 1, the dynodes 16 in each row 12, 14 are arrangedon the members 26, 32 such that the flanges 20 are in each case closestto the input end 10a of the multiplier 10 and the flanges 22 closest tothe output end 10b thereof. The flanges 20, 22 thus extend from thesurface 18 of the dynode of which they form part inwardly towards thedynodes of the opposed row and towards the input end of the multiplier.The arrangement results in the flange 22 of each of the dynodes of eachrow, save for the last or output end dynodes in each row, being adjacentand parallel to, and spaced from, the flange 20 of the immediatelyadjacent dynode.

At locations towards the output end of the multiplier, the members 26,25 are secured together by means of a screw 40 which passes throughopenings in the members 26, 32 and through an intervening spacer member44 formed of insulating material. Screw 40 has a nut 47 on the endthereof opposite the screw head and the nut and screw are tightened upto hold the members 26 and 32 firmly in position so as to extend inparallel cantilevered relationship away from the member 44. Thepositioning of the last-mentioned holes in the members 26, 32 is suchthat the dynodes in row 12 are not positioned in direct opposition tothe dynodes in row 14, but are displaced by a distance equal to one-halfof the pitch distance between dynodes in each row, reckoned in thedirections of extent of the rows.

Row 14, by virtue of the above-described arrangement, has an input enddynode 16, designated by reference 16' which is furthest from the outputend of all dynodes in that row and is the first of the dynodes 16 in thearray. As mentioned, there is, however, an input dynode 46 forming partof row 12, and it is this dynode which first receives input signal inuse of the multiplier. Dynode 46 is of inverted U-shaped configurationformed by folding of a generally rectangular blank so as to present atop planar portion 48 which extends transversely across the input end ofthe multiplier and two opposed planar arm portions 49 and 50. Armportion 50 is at an angle of approximately 90° to portion 48 and carriesa strap 52, like the straps 28 of the dynodes 16, by means of which thedynode 46 is secured to insulating member 26. Arm portion 49 extends atan angle of approximately 45° to portion 48 and, in position as shown inFIG. 1, extends from the free edge of portion 48 remote from flange 50,and from a location well outboard of member 32 and dynodes 16 of row 12,towards the output end 10b of the multiplier and inwardly to terminateat a location immediately adjacent, but spaced from, the junctionbetween the flange 20 and the surface 18 of the first dynode 16 (shownby referenced numeral 16" in FIG. 1) in row 12. Portion 49 has a dynodesurface 54 at the surface thereof which is directly opposed to flange50.

Portion 48 of dynode 46 has a central aperture 60 (FIG. 3) which iscovered by fine "micromesh" 61 (Trade name for E.M.I. fine mesh)(preferably having 100 meshes per 25 millimeters lineal). As describedlater, electrons or other charged particles can enter the multiplier viaaperture 60 to impinge on surface 54 for direction on to dynode 16'.

Dynode 46 also carries two opposed electrostatic shields 59 formed ofmesh material which close opposed sides of the electrode.

Towards the output end of the multiplier, the member 26 carries anadditional dynode 58, forming part of row 14 and which differs in formfrom the dynodes 16. Dynode 58 has an intermediate portion 62 and twoopposed flanges 64, 66 formed by folding of a metal blank. Flange 64 isof the same form as flanges 20 on dynodes 16, and portion 62 defines adynode surface 63 similar to dynode surfaces 18. The flange 66 is longerthan flange 64 when viewed from the side as in FIG. 1 and of length,measured away from the junction with portion 62, which is only slightlyless than the distance between the two rows 12, 14 of dynodes. Flange 64extends at an angle of about 45° relative to portion 62. As shown inFIG. 1, the dynode 58 is positioned so that the dynode surface 63thereof is located so as to constitute an extension of the row of dynodesurfaces 18 on dynodes 16 in row 14 and with the flange 64 arranged toextend in parallel relationship but spaced from the flange 22 of theimmediately preceding dynode 16 in row 14. The flange 66 of the dynode58 extends transversely across the space between the two rows ofdynodes, from the row 14 to a location just short of the member 32.

The location of flange 66 is such that it is positioned further from theinput end of the multiplier than the flange 22 of the last dynode 16"'in row 12. Between the last dynode 16"' in row 12 and the flange 66,there is positioned a collector 70 of mesh material which is secured tomember 32 to extend transversely of the multiplier and in generallyparallel overlying relationship to the flange 66.

Collector 70 and dynode 58 are held on to respective members 32, 26 bystraps 72, 74 respectively, in the same manner as the dynodes 16 areheld to the members 32, 26.

The mode of electrical interconnection of the dynodes 16, 46 and 58 isshown in FIG. 4. The dynodes 16 and 58 of row 14 are interconnected toeach other and across an electrical supply (not shown) by a chain 75 ofresistors, the resistors between each pair of dynodes being of value "R"and those between the supply and the dynodes 16' and 58 being of valueR/2. Similarly, the dynodes 16 and 46 of row 12 are interconnected toeach other and across the supply by a chain of resistors 77 includingresistors each of value "R". The end resistors of chain 77 connect theend dynodes 46, 16'" of row 12 to the supply. The resistors in chains 75and 77, save for those marked "α" "β" and "γ" in FIG. 4 are shown byreference numerals 76 in FIG. 1, being mounted on the multiplier itself.The resistors of the chains 75 and 77 marked "α" "β" and "γ" areexternally provided, although of course this is not essential. Thecrimped together ends of the arms 28a, 28b of the straps 28 andcorresponding crimped together ends of the straps associated with dynode46 and dynode 58 are used as terminal posts for electrical connection ofresistors 76.

In use, an electrical potential is applied across the two chains inparallel so as to apply to the dynode 16, the dynodes 46 and 58 and thecollector 70 electric potentials which gradually become more positivefrom the dynodes 16" and 46 along each row to the collector. Forexample, as shown in FIG. 4, a negative potential of 3000 volts may beapplied to dynode 46 and zero potential may be applied to the collector70. FIG. 4 is to be taken as a potential diagram illustrating the "rest"potentials applying in use of the multiplier. As is well known inconventional practice a substantial impedence Z must prevail between thecollector 70 and the zero or ground potential, across which impedencethe output of the multiplier is developed. The arrangement of the valuesof the resistors as described above results in the first dynode 46 beingat a somewhat greater negative potential than the first dynode 16' inthe row 14 and, viewed along the length of the multiplier, there isdefined a path of gradually more positive going potential from onesuccessive dynode to the next through the multiplier with successivedynodes being alternately in row 12 and row 14. This results in adistribution of electrical charge between the successive dynodes and, asshown in FIG. 7, this electrical charge acts to focus secondaryelectrons from successive dynodes in the multiplier on to the nextfollowing one until the last dynode surface 63 is reached, from whichsecondary electrons are passed to collector 70. Charged particles, forinitiating a charge current through the multiplier can pass into themultiplier through aperture 60 in dynode 46.

The described configuration for the dynodes has been found to beparticularly satisfactory in use and provides good focusing of secondaryelectrons through the multiplier. Departures in the relative angle ofthe flanges of the dynodes from the 45° angle mentioned to anglesbetween 40° to 50° do not cause serious impairment of this focusingability.

The dynode surfaces of the dynodes may be formed in a conventionalfashion such as from beryllium-copper or silver-magnesium materialtreated in accordance with usual practice. Alternatively, they maycomprise aluminium containing materials. The material, whether it bealuminium containing material, beryllium-copper or silver-magnesium maybe in the form of a flexible foil 73 applied over the dynodes and heldin place such as by clips 75 as shown in FIGS. 9 and 10. The clips 75are not, however essential, in most instances merely folding margins ofthe foil back around the opposed edges of the dynode portions 17 issufficient to hold the foil in place.

Surprisingly, it has been found that commercial aluminum foils form goodsecondary emission materials. Particularly, referring to FIG. 12, thereis shown therein a plot of secondary electron yield (δ) against incidentenergy for three emissive materials. The secondary electron yield is theratio of charge current leaving the emissive surface to the incidentcharge current. Generally, the yield varies over a range of incidentenergies.

In FIG. 12, the plot C is a plot of secondary electron yield againstincident energy for a commercial beryllium-copper emissive material. Theyield δ exhibits a peak in the range 200-300 eV at which δ has amagnitude of about 2.4. Plots A and B in FIG. 12 are plots for twoaluminium foil emissive materials. The materials were commerciallyavailable aluminium foil of thickness 0.017 mm, being that marketed inAustralia by Comalco Limited bearing international registereddesignation 1145. This foil is formed by rolling a double sheet,resulting in a highly polished roller contact surface and a dullaluminium to aluminium surface for each of the separated sheets. Afterrolling, the material is, during manufacture, heat treated at 350° C.for between one-half an hour and two hours.

The plot A of FIG. 12 represents the variation of yield δ againstincident energy for the aforementioned aluminium foil when the highlypolished surface thereof was used as the emissive surface whilst plot Brepresents the corresponding variation for the material when the dullsurface was used as the emissive surface. Thus, the highly polishedsurface exhibited greatly increased peak yield (of approximately 4) ascompared with the aforementioned beryllium-copper material whilstsubstantial improvement over the beryllium-copper material was alsoachieved using the dull surface. However, with the dull surface used,the peak yield obtained was somewhat less than that obtained using thehighly polished surface, being of the order of 3.8.

The arrangement of the multiplier of FIG. 1 can be varied to form acylindrical electron multiplier such as the multiplier 115 shown in FIG.8. In this multiplier, the dynodes 90 in one row are cylindrical, thedynode surfaces 92 being defined on an outer surface thereof, whilst thedynodes 94 in the other row are cylindrical with dynode surfaces 96defined on inner surfaces thereof. The two rows are arranged coaxiallly.Each dynode has an annular flange 98 or 102 at one end and an annularflange 100 or 104 at the other end these representing a configuration,when viewed in axial section of the multiplier, corresponding to theside configuration of the flanges 20, 22 of the construction of FIG. 1.An annular collector 110 is provided at the output end of the multiplier115. In use, then, the multiplier of FIG. 8 functions in the samefashion as that shown and described previously in relation to multiplier10, the path of charge current through the multiplier being as shown byarrows 108 in FIG. 8.

FIG. 11 is a diagram showing a still further modification in which amulti-channel multiplier 135 is constructed by using dynode formingbodies 140, each body having a planar portion defining two dynodesurfaces 18, one to either side and having end portions 142; 144, eachdefining a respective pair of flanges 20;22. Paths of charge currentflow through the parallel multiplier channels defined by the bodies 140are designated by reference numerals 146.

I claim:
 1. A dynode array for an electron multiplier of the kind inwhich a charge current is amplified by passage to, and by secondaryemission of electrons from, surfaces of successive dynodes of the dynodearray, there being two generally parallel rows of said dynodes in saidarray, the dynodes in each row being in side-by-side position, and saidsuccessive dynodes being dynodes in alternating ones of said rows, eachsaid dynode being shaped such that electric potentials which are appliedto the dynodes of the array generate an electric field between the tworows such as to effect substantial direction of secondary electronsproduced at each said surface to the surface of the next successivedynode; in which said dynodes in one said row have the said surfacethereof facing the other said row of dynodes and those of the other saidrow have the said surface thereof facing said one row; each said dynodehaving first and second flanges positioned at opposite sides of saidsurface thereof, said first and second flanges extending transverse tothe directions of extent of said rows, the flanges of dynodes in saidone row extending from the surfaces of those dynodes towards the othersaid row and the flanges of dynodes of said other row extending from thesurfaces of those dynodes towards said one row, and each adjacent pairof said dynodes in said one row and in said other row having the firstflange of one dynode of the pair adjacent and spaced from the secondflange of the other dynode of the pair, and the first flange of eachsaid dynode being closer to an input end of the array than the secondflange of that dynode; the surfaces of dynodes in said one row and ofdynodes in said other row being linear and parallel to the lengthwisedirection of extent of the respective row, when the array is viewed inlengthwise cross-section normal to tangents to said surfaces at thelocation of the cross-section; characterized in that said first flangeof each said dynode extends at an obtuse angle to the surface of thatdynode, to extend away from that surface towards said input end, and thesecond flange of each said dynode extends at an acute angle to thesurface of that dynode, from the junction of the flange with thatsurface towards said input end, said first flange of said one dynode ofeach adjacent pair of said dynodes being generally parallel to thesecond flange of the other dynode of the pair.
 2. A dynode array asclaimed in claim 1, wherein said flanges, when viewed in saidcross-section, extend at an angle in the range 40° to 50° to thedirections of extent of said rows.
 3. A dynode array as claimed in claim1, wherein said first flanges are longer, when viewed in saidcross-section, than the second flanges.
 4. A dynode array as claimed inclaim 2, wherein said first flanges are longer, when viewed in saidcross-section, than the second flanges.
 5. A dynode array as claimed inany one of the preceding claims wherein said surfaces and said flangesare generally planar.
 6. A dynode array as claimed in any one of claims1 to 4, wherein said surfaces and said flanges are generally annularwith the dynodes of said one row having said surfaces in coaxialrelationship to said surfaces of the dynodes of the other row.
 7. Adynode array as claimed in claim 1, wherein the dynodes of said one roware fixed in spaced relationship along the length of a first insulatingmember, and the dynodes of said other row are fixed in spacedrelationship along the length of a second insulating member, saidinsulating members extending in parallel spaced relationship and in thedirections of extent of the respective rows.
 8. A dynode array asclaimed in claim 7, wherein each dynode is affixed by a flexible strapto its respective insulating member.
 9. A dynode array as claimed inclaim 8, wherein said surface and the flanges of each dynode are formedon a respective dynode structure having a said strap thereof secured toa face of the structure opposite the said surface, said straps of eachdynode extending around the said respective insulating member.
 10. Adynode array as claimed in claim 9, wherein each said insulating memberis elongate and generally planar with the dynodes affixed theretoarranged so that the said opposite faces thereof are positioned inoverlying relationship to one major face of the respective insulatingmember, the said straps being interposed between said opposite faces ofthe dynodes and the said one face of the respective insulating member,and defining strap arms extending oppositely from each other from therespective said opposite dynode face and around opposed edges of therespective insulating member to strap arm ends secured together at asecond face of the respective insulating member which is opposite saidone face.
 11. A dynode array as claimed in claim 9, wherein theinsulating members have opposed notch portions along the edges thereofto receive and locate the said straps where these pass around the edgesof the insulating members.
 12. A dynode array as claimed in claim 7,wherein the two insulating members are secured together at respectiveadjacent one ends thereof only.
 13. A dynode array for an electronmultiplier of the kind in which a charge current is amplified by passageto, and by secondary emission of electrons from, emissive surfaces ofsuccessive dynodes for the dynode array, each said dynode being shapedsuch that electrical potentials which are applied to the dynodes of thearray generate an electric field between successive dynodes such as toeffect substantial direction of secondary electrons produced at eachsaid emissive surface to the emissive surface of the next successivedynode wherein each said dynode is in the form of a rigid conductivesupport structure provided with means for making electrical connectionto the respective dynode, and an electrically conductive flexible foildetachably mounted in coplanar relationship with an underlying portionof the support structure, said foil defining, at an outermost surfacethereof, the emissive surface of the respective dynode, each saidsupport structure further defining at least one further rigid portiondefining, for the associated dynode, a conductive surface extending atan angle to the emissive surface of that dynode to focus electrons ontothat emissive surface.
 14. A dynode array as claimed in claim 13 whereinsaid emission surface is planar and margins of each foil are wrappedaround respective opposed edges of said underlying portion of therespective dynode structure to locate the foils in position.
 15. Adynode array as claimed in claim 13 wherein said foils are substantiallyaluminum foils.
 16. A dynode array as claimed in claim 15 wherein saidaluminum foils are aluminum foils meeting the specification ofInternational registered designation
 1145. 17. A dynode array as claimedin claim 15 wherein said aluminum foils are formed from rolled sheets ofaluminum material which have been heat treated at substantially 350° C.for a period between one and one half and two hours.
 18. A dynode arrayas claimed in claim 17 wherein said foils have a composition complyingwith International registered designation
 1145. 19. A dynode array isclaimed in any one of claims 15 to 18 wherein said aluminum foil has apeak secondary electron yield, δ, of substantially 3.8 or more.